The present invention relates to a method for producing a hot-dispersible HMT-starch with delayed thickening and to the starch so produced.
The term "HMT starches" is applied by those in the art to starches which have been subjected to a heat and moisture treatment. In contrast to other physical treatments (as for instance in the case of production of pregelatinized cold swelling starches), in such a purely physical treatment the properties of the starch (ability to swell, dispersibility etc.) is modified in such a way that no gelatinization or any other recognizable change of the starch granules, is detectable (for instance the typical birefringence of a native starch granule is completely retained when such HMT starches are examined under a polarizing microscope).
Significant changes in the starch properties can be obtained by a heat-moisture treatment of those starches showing a B or C type x-ray pattern. A typical B type starch is potato starch, which therefore is particularly suited for a heat-moisture treatment.
In the case of potato starch, the original properties are substantially modified by such a heat-moisture treatment. For instance, untreated potato starch yields a transparent clear paste with a stringy, sticky consistency after gelatinization while a heat-moisture treated potato starch forms short, opaque pastes and on cooling even moldable gels. Whereas natural starch forms lumps when added to boiling water, heat-moisture treated starch is hot dispersible, and, although natural potato starch thickens as soon as the gelatinization temperature is attained, in the case of a heat-moisture treated starch, the gelatinization is delayed and in some cases may even take place at temperatures above 100.degree. C.
The essential parameters determining the degree of heat-moisture treatment are the moisture content of the starch, the temperature of treatment and the time of treatment. In general it can be said that the higher the moisture content of the starch, the higher the degree of modification. However, moisture contents above a certain level limit the maximum possible processing temperature because the starch granule will be gelatinized. In the same way an increase in temperature increases the degree of modification, too high a temperature not only involves the risk of gelatinization but also the risk of thermal degradation of the starch (the so-called dextrinization). Similarly, prolonged treatment times lead to higher degrees of modification.
In the processes known up to now in which aqueous starch slurries are used, heating is only possible up to a temperature just below the gelatinization temperature. This requires careful control of temperature and time in order to exclude any gelatinization of the starch, as well as resulting in long treatment times of up to several days. For this reason it is preferred to reduce the moisture content to such a degree that the starches can be treated at temperatures above the normal gelatinization temperature without gelatinization. Such processes are called "semi-dry processes", and substantially shorten the treatment time. The general aspects of heat-moisture treatment of starches are described in detail in "Heat-Moisture Treatment of Starch" in Cereal Chemistry 44, (1967) 8-26. In addition, a whole series of patents are available which describe various forms of heat-moisture treatments, the majority of which are semi-dry processes.
German patent specification no. 2,930,664 provides a very detailed description of the problems which are involved in semi-dry processes for the manufacturing of HMT starches. The patent relates to a food product which contains a thickening agent consisting of an HMT root and/or tuber starch. The starches used for this purpose are treated in a semi-dry process at 103.degree. C. for 115 minutes or, respectively, at 100.degree. C. for 195 minutes. Some patents using the semi-dry principle recommend the addition of emulsifiers (European patent 76381, U.S. Pat. No. 184,527) or, respectively, the use of microwaves for the transfer of heat (European patent 150,715 and U.S. Pat. No. 4,508,576).
In processes operating in an aqueous suspension, the main emphasis is placed on the suppression of the gelatinization which can easily occur in the presence of excess water. In U.S. Pat. No. 3,977,897 the addition of salts, such as sodium or magnesium sulfate, is recommended in order to increase the gelatinization point, but such salts have to be removed by intensive washing before the starches can be used in foodstuffs. By using very high salt concentrations it is possible to apply temperatures up to 100.degree. C. which, of course, remarkably reduce the treatment times.
In U.S. Pat. No. 3,583,874 and European patent no. 110,549 the gelatinizing effect of water during the heat-moisture treatment is suppressed by a sufficiently large addition of organic solvents, miscible with water (e.g. alcohol). Such methods are not only technically complicated but are expensive so that they have not been proposed for the HMT starches set forth herein, but only for the production of special cold swelling starches. In said processes the removal of the solvent is always the problem, which, of course, is necessary before such starches can be used for foodstuff applications.
Processes in which an effective heat-moisture treatment is performed in a short treatment time in an aqueous suspension without suppression of gelatinization, have not yet been described.